This invention relates to cylinder liners, and in particular to cylinder liners which are designed as wet liners with a mid-stop flange incorporated into the liner.
For convenience, in this specification and the accompanying claims, in an engine block each constituent cylinder liner in accordance with the present invention is considered to have an axis of symmetry which extends vertically above a co-operating crankshaft. The end of the liner remote from the crankshaft is referred to as the top or upper end, and the other end is referred to as the lower or bottom end. Approximately from the level of the mid-stop flange to the upper end is considered to extend the outer portion of the liner, and the remaining, inner portion of the liner is considered to extend from said mid-stop flange level to the bottom end of the liner. However, it will be understood that references describing the relative positions of the ends, and of the two constituent portions, of any arrangement within an engine block of a liner in accordance with the present invention, are included in the corresponding references given above with respect to the arrangement of a liner extending vertically above the crankshaft.
Further, the term radial, or radially-extending, is used in respect of any distance, or feature, extending in any direction in a plane at right angles to the axis of the cylinder liner; and the term axial, or axially-extending, is used in respect of any distance, or feature, extending parallel to the liner axis.
A cylinder liner of the general type to which the present invention relates is described and claimed in UK Patent No. 2 035 452. This specification describes a cylinder liner which has, in combination, a series of features which include a top boss on the liner the top boss having an axially extending cylindrical surface forming an interference fit with the engine block when in position; a mid-stop flange with a surface which extends radially outwardly from the liner and engages a complimentary radially extending ledge in the engine block; an upper portion between the top boss and the mid-stop flange which forms a wall of a coolant passage, the axial length of which passage wall is not more than 30% of the axial length of the liner; and an inner portion which has an axial length of at least 30% of the axial length of the liner and which is wholly free of direct supporting and heat conducting contact with the engine block.
In practice this liner design suffers from the disadvantage that lack of radial support at its bottom end allows the inner portion of the liner to ovalise during use to an undesirable extent, and under extreme conditions this is liable to lead to cracking of the liner.
From a practical point of view, however, it is difficult to provide an adequate support at the bottom end of the liner since this end of the liner is to be inserted into the block first, and the liner has an interference fit on the top boss, as stated above.